We will analyze the immunological basis of host-tumor interactions from a multifaceted approach. Tumors, as immunogens, (1) are able to exert direct nonspecific influences on the immunological apparatus; (2) are capable of qualitative and quantitative change in antigen expression employing developmental processes which parallel progression; (3) are extremely close to "self" with regards to membrane-associated structures; and (4) may possess the necessary requisites to actually dictate the type and intensity of immunological responses being induced to their antigenic determinants. The immunological system, with its complex network of antigen-driven pathways, is also dynamic with regards to the nature of eventual outcomes. The final products of the numerous available mechanisms can function either synergistically or antagonistically with respect to one another. In response to the tumor-specific antigen stimulation, either suppressor or effector responses can dominate. It is imperative to consider the immunological status of the host concomitantly with the potential influences being simultaneously exerted on the system by the tumor as well as the environment. We will investigate whether Langerhans cells are involved in the ultraviolet light-mediated induction of suppressor T cells; evaluate the possible relationship between the major histocompatibility complex and ultraviolet light-mediated immunoregulatory effects, and the heterogeneity of the nature and specificity of mechanisms which regulate antitumor responses; analyze the antigenic specificities and tumorigenic properties of individual cells which collectively comprise a primary tumor mass; and initiate a search for phototoxic chemicals which are capable of exerting secondary immunoregulatory influences similar to UVB or 8-methoxypsoralen plus UVA exposure.